The Sound of Recycling

Abstract

This thesis explores the upcycling of post-consumer glass waste into functional acoustic panels for architectural applications. Addressing both environmental concerns and indoor acoustic performance, the research investigates the potential of glass foaming and fusing techniques to transform discarded glass, such as soda lime, light bulb glass, and mixed cullet, into porous, sound-absorbing elements suitable for use in the built environment.
The study begins with a comprehensive literature review on glass recycling, classification of glass waste streams, and limitations within current recycling infrastructures, particularly regarding contaminated or mixed glass cullet. The material science behind foam glass production is examined, with a focus on how porosity, pore size distribution, and pore interconnectivity affect acoustic absorption. Kiln-based secondary casting and fusing techniques are also reviewed as accessible, energy-efficient alternatives to primary glass forming processes. In parallel, the acoustic literature is reviewed to understand key performance indicators such as reverberation time, clarity index, and sound absorption coefficient, especially in relation to porous materials. Furthermore, the review introduces the growing use of computational tools in acoustic design, including the use of simulation environments (such as CATT-Acoustic) and parametric optimization workflows (e.g., in Grasshopper), which allow for data-driven decision-making in early-stage material and geometry development.
A series of experimental trials were conducted to evaluate how various glass types respond to foaming, with particular attention paid to their level of contamination. Variables such as foaming agent type (calcium carbonate, eggshells, manganese dioxide), glass composition, particle size, and firing schedule were tested for their influence on pore development and structural integrity. Glass was successfully foamed at 860°C and 790°C. Notably, eggshells showed strong compatibility not only with clean soda lime cullet, but also with more contaminated light bulb glass and mixed cullet, yielding homogeneous pore structures. Re-foaming tests and prototype casting using 3D-printed moulds demonstrated the adaptability and scalability of the method. Selected samples were tested using an impedance tube, confirming their ability to absorb mid- to high-frequency sound, particularly around 1000 Hz, a range critical to both music and speech. The highest absorption coefficients were achieved with soda lime glass; however, the presence of contamination in other glass types did not significantly diminish acoustic performance of the developed material.
To enhance mechanical integrity, fusing trials were conducted to bond the porous layer to a solid glass surface. This was done without compromising the internal pore structure, achieving successful fusion at 705°C for samples made of both light bulb and soda lime glass paired with soda lime float glass.
To evaluate architectural performance, the panels were applied in a digital acoustic simulation of a real-world case study: the 2200 m³ Theatre Hall at TU Delft. The space suffers from excessive early reflections and high clarity index values. Field measurements were used to calibrate a model in CATT-Acoustic, which then informed a parametric optimization process in Grasshopper to determine optimal panel placement. The strategy focused on reducing C80 while preserving reverberation time, maintaining suitability for both rehearsal and performance scenarios. Although improvements in acoustic metrics were modest, they demonstrated that the porous panels successfully reduced problematic reflections without overly damping the space.
Through the integration of material science, real-world measurement, computational acoustics, and parametric design and design optimization, this thesis presents a viable, circular strategy for converting low-value glass waste into functional acoustic components. The findings point to broader potential applications in performance spaces where a nuanced balance between clarity and reverberation is required, offering a solid foundation for further research into sustainable material acoustics.